The present invention relates to an air-fuel ratio feedback control system of an electronic fuel supply control apparatus, in which the output of an oxygen sensor disposed in an engine exhaust manifold is fed back to control the time width of a supply pulse for determining the amount of fuel supply and to control the air-fuel ratio constant, more in particular to such an air-fuel ratio feedback control system effectively used with a three-way catalyst for purification of the exhaust gas.
Conventional systems of this type comprise an oxygen sensor for detecting the air-fuel ratio from the oxygen of the exhaust gas, or especially, the oxygen concentration of the exhaust gas of the engine, a comparator circuit for determining whether or not the air-fuel ratio is higher than a stoichiometric air-fuel ratio on the basis of an output signal from the oxygen sensor, a delay circuit for delaying the output signal of the comparator circuit for a predetermined time, and an integrator circuit for performing an integration in accordance with the output of the delay circuit, whereby the air-fuel ratio is corrected to the stoichiometric value, thus improving the purification rate of the three-way catalyst. Conventionally, the air-fuel ratio has been generally feedback controlled to rich or lean side by the delay circuit thereby to attain a high purification of the exhaust gas composition under the steady and transient operating conditions of the engine.
It has been found, however, that the delay time of the delay circuit has a close relation with the operating conditions of the engine or specifically with the amount of the intake air into the engine and the engine rotational speed. The optimum delay time required of the delay circuit under a high-load and high-speed condition where the amount of intake air for each revolution of the engine is great is different from that under the idling or decelerating condition where the amount of intake air for each revolution of the engine is small. If the delay time is unnecessarily long, the error between the actual air-fuel ratio and the stoichiometric air-fuel ratio becomes excessive.